Auto Focus giving me problems

[LuckySe7en]

I'm not sure what it is but the last few times I've been shooting, the auto focus on my 18-55 has been acting up. I've noticed it in low light to dark situations. I've even put the camera on auto mode, shot my subject and find out that the background is in focus and not the people. It happens a lot. I have a canon xs, does anyone know whats going on?

 

[johnh2005]

Try posting some of the shots you are having problems with. I am sure there are some really smart people here that can look at the exif (?) info and probably tell you what is going on. It sounds to me like you are not setting your auto focus position properly. If you do not know how/don't feel like changing the auto focus position then you will need to center your target press the shutter button half way to focus and meter then while holding down the button recompose and shoot.

 

[KmH]

Auto focus needs good contrast to work right. Your lens is auto focusing, but not where you wanted it to focus.

In low light situations there often isn't enough light for AF to work well.

You don't mentionm what camera/lens combo you were using, but most DSLR cameras have an AF assist fature for use in low light.

AF systems also won't work well if your max lens aperture is at or near f/5.6.

There are 2 kinds of auto focus sensor in most cameras: regular and cross-type.

Regular focus point sensors can only detect contrast horizontally or vertically, depending on the orientation of the sensor. Cross-type sensors can detect contrast both horizontally and vertically and are usually a more accurate and sensitive sensor.

Look in your camera users manual for more information regarding the auto focus system in your camera.

 

[Big Mike]

What focus mode and point selection mode are you using?

The default is for all of the AF points to be active...but I'd suggest selecting just one, so that when you focus on something, you can choose exactly what part of the image you are focusing on.

Also, what Focus mode are you using? One Shot, AI Servo or AI Focus?

 

[LuckySe7en]

camera/lens combo is Canon Rebel XS w/kit 18-55
I'll post the pics tomorrow but I set it in full auto mode. I'm pretty positive my setting is one shot. Point selection is set to all of them.
If it's any help, this only started happening after I attached a new flash. Purchased a promaster 7200.

 

[shmne]

The easiest fix to bad auto-focus in low light situations is bringing a small hand held high power LED light. I always have one in my pocket, I hold it similarly to how police hold their gun / flashlight. Flash it on while it focuses, then let go of the thumb button right as you shoot. If you're using flash you don't even need to turn it off necessarily.